Peter Calvocoressi: Political writer who served at Bletchley Park and assisted at the Nuremberg trials

Peter Calvocoressi was a senior intelligence officer at Bletchley Park who was picked soon after the war to lead a team of experts on the German military at the Nuremberg war crimes trial.

For almost six decades he was a distinguished author, urbane publisher, and human rights activist, but those two early activities – Bletchley Park cryptanalysis and Nuremberg prosecutions, military realities and an idealistic claim to justice – framed his long and diverse career.

He was a child of the Greek diaspora, born in Karachi in November 1912, to parents descended from mercantile families from Chios. When Peter was three months old his father was assigned to Liverpool, where his parents raised him and two sisters in a cosmopolitan cocoon. He grew up with Greek friends, spoke French at home as well as English and had family trips to Italy and Greece.

A King's Scholar at Eton, he focused on history and German. In 1931 he moved on to Balliol, having been passed over for a scholarship in favor of the future historian and master Christopher Hill. While in Oxford Calvocoressi avoided politics, later saying he never even knew the location of the Oxford Union. He focused on Modern History, and had as tutors V. H. Galbraith and Sir Lewis Namier, whose knowledge of European politics characterised his own later writings.

Leaving Oxford with a First, Calvocoressi went to London with a view to joining the diplomatic service. Learning that his father's French birth would preclude him, father and son secured an audience with Anthony Eden, then a minister in the Foreign Office. Eden confirmed his ineligibility and added that with so distinctive a Greek name he could never rise to the top. Calvocoressi turned to law. He joined an equity chambers in Lincoln's Inn. Shy but a good dancer, he met Barbara Eden, daughter of Lord Henley, and in 1938 they married.

Everybody at Bletchley Park had a story about how they were plucked from classics or mathematics. Calvocoressi was serving in the Ministry of Economic Warfare reviewing shipping manifests. In 1940 he tried to volunteer for the War Office but was rejected because of a head injury he had sustained in a car accident in which his mother died. He took umbrage at the reason listed on his file: "No good, not even for Intelligence." Through a connection he applied to the Air Ministry and was accepted by the Intelligence division. He was sent to RAF bases in Northumberland to brief pilots but soon recalled to London and sent to Bletchley Park.

Calvocoressi was assigned to Hut 3A to work on "Ultra," the decodes of Luftwaffe messages sent over Enigma machines. Luftwaffe units relied more than other services on Enigma, and their principal cipher, Red, "was broken daily [without interruption from May 1940], usually on the day in question and early in the day. Later in the war I remember that we in Hut 3 would get a bit techy if Hut 6 had not broken Red by breakfast time."

Calvocoressi's job was to study decrypted messages and fill in gaps, decide their meaning, weigh their urgency and decide on the recipients. In a typical eight-hour shift, Hut 3 might produce 30 decodes. For most of the war the Hut was led by Eric Jones, a Midlands manufacturer who went on to a brilliant career in intelligence, with E.J.B. ("Jim") Rose as the head of 3A and Calvocoressi his deputy.

Calvocoressi impressed colleagues as competent, less ebullient than Rose but calm and erudite. When American cryptanalysts were introduced in mid-1943, he worked easily with them and formed lifelong friendships with their chief, Colonel Telford Taylor, Robert Slusser, later a leading Sovietologist, and eventual Supreme Court Justice Lewis Powell. After lodging in a series of unsatisfactory billets, Calvocoressi bought the roomy 18th-century Guise House. For the rest of the war he hosted seven or eight personnel, including Americans, giving preference to those who could play musical instruments.

Leadership of Hut 3A brought special responsibilities. After the setback in the Ardennes in December 1944, Calvocoressi and the renowned Cambridge classicist F.L. Lucas, later President of the British Academy, were assigned to investigate if Ultra had failed or if its recipients in the SHAEF and Army Group headquarters had failed to use it. He concluded that Bletchley Park had accurately outlined German preparations since August, though not the timing, and that commanders had been lulled into thinking too far ahead. Lucas and Calvocoressi "expected heads to roll at Eisenhower's HQ but they did no more than wobble."

A more difficult question was presented when the former 3A chief Jim Rose, now at the Air Ministry, heard of plans to bomb Dresden. He went to the US commander Carl Spaatz, who agreed to cancel the raid if evidence revealed no military targets, and Air Marshal Harris agreed. Rose called Calvocoressi, his successor in Hut 3A, who stressed that Panzer units were not being routed near Dresden and that the city should not be bombed. But British Bomber Command spurned Rose and the Calvorocessi estimate, with terrible consequences.

After V-E Day Calvocoressi stood in the General Election as Liberal candidate for Nuneaton, a Labour seat where he increased his party's tally but finished third.

Days later he received overtures from his friend Taylor, now working on the American case for Nuremberg. US planners had realised that direct evidence might be lacking against some perpetrators and that the Nazi state operated through institutions as well as individuals. The answer to both issues was a proposal by Colonel Murray Bernays to charge organisations as well as leaders; if an entity were found guilty, prosecutors could then charge leaders with criminal membership if murder charges were impossible. Most planners assumed that the guilty would include the military high command as well as the Party and SS. Taylor convinced his chief, Supreme Court Justice Robert Jackson, that they presently lacked evidence against the military but that Calvocoressi was the ideal man to find it.

In January 1946, with Calvocoressi at his side, Taylor presented the case against the High Command, using testimony from SS General Erich Bach-Zelewski, who led anti-partisan campaigns. One historian later complained that the two men went easy on Bach-Zelewski, but contemporaries praised them for Taylor's eloquence and because he and Calvocoressi had a major perpetrator testify without promising immunity or compromising later charges.

Later, Taylor was chosen to lead further American trials in Nuremberg. He turned again to Calvocoress. Taylor sent him and three researchers to Washington, where for months he supervised the archiving of captured military records. The evidence they processed provided the basis for convictions including those involving atrocities in the Balkans and Norway.

The plan, was that the British would also commence major trials but they balked, yielding to Foreign and War Office pressure and rejecting Shawcross's hopes, American overtures and Calvocoressi's evidence.

Now came the question of a peacetime career. Calvocoressi wanted work involving public affairs, but a brief return to the bar and his parliamentary defeat convinced him to find a different route. For a few years he ran a fledgling Liberal International but found it unworkable. He wrote his first book, a survey of Nuremberg that is more thoughtful than most. An offer came to lead a new press organisation based in Geneva, but he turned it down, recommending Jim Rose.

In 1949 Calvocoressi joined the Royal Institute of International Affairs, the think tank in Chatham House founded by Arnold Toynbee. Toynbee gave him responsibility for the annual Survey of International Affairs, meaning that he wrote a judicious but opinionated volume each year surveying world events. The experience taught him to write accurately, elegantly, and fast.

After five years and 10 hefty volumes, Calvocoressi left Chatham House and became a partner at Chatto & Windus, which had published his Nuremberg book. Calvocoressi helped build up its annual turnover from £200,000 to £500,000. It had absorbed the Hogarth Press, whose guiding spirit was Leonard Woolf. Calvocoressi came to admire Woolf deeply, calling him "the only man I ever met who seemed to me to be right about everything that mattered" and taking to heart the older man's editorial maxim that "there never was a book which could not be improved by cutting."

He was writing extensively on international relations – a weekly column for provincial newspapers begun while he was at Chatham House led to a book on Suez and a follow-up 10 years later; a monograph on the public reaction to the Sharpeville massacre; and a general study of world order in the age of decolonialisation. He left Chatto & Windus in 1966 for a Readership in International Relations at Sussex. Teaching gave him enormous satisfaction, but when the opportunity arose five years later to become editorial director of Penguin Books, he could not resist.

The challenge was to strengthen its position, as Calvocoressi said, as a place "for top people to write little books about big subjects for large audiences". But his tenure came to grief when Pearsons bought Penguin with Calvocoressi's support ("perhaps the biggest misjudgment of my life") then squeezed him out. He resumed writing and was hired by the Open University, which saw the possibility of a university press that might turn the material prepared by its faculty into books for the public. Calvocoressi supervised the effort, leaving with an honorary degree in 1990.

From 1962 to 1971 he was a member of the UN Sub-Commission on the Prevention of Discrimination and Protection of Minorities. His Balliol friend David Astor asked him in 1963 to chair the Africa Bureau, a study group, and like Rose, Calvocoressi was soon writing about apartheid and racism.

From 1961 to 1971 he was on the Council of the International Institute for Strategic Studies founded by his friend Alistair Buchan, and he was twice invited to lead Chatham House. Brought to Amnesty International to mediate an internal dispute, he served on its executive board from 1969 to 1971. And he found time for political campaigns with a more personal connection: he commissioned, and wrote a sympathetic introduction to, an account of Zionist settlers who were British spies in the First World War. He protested against the 1967 coup of the Greek Colonels. But at least as important to the long-time editor must have been his tenure as chair of the London Library from 1970 to 1973.

He continued to write prolifically – his survey of the Second World War is regarded, along with Gerhard Weinberg's, as the best general study of its kind; an elegant account of Ultra intelligence at Bletchley Park; and a "Who's Who" of characters in the Bible that surprised friends who knew of his fierce atheism. He wrote a foreign-relations study of Africa, a study of peace from the Gospel to the UN, and a study of Europe's terrible 20th century and improving prospects with the end of the Cold War. Each book showed his breadth, erudition, concision – his lesson from Leonard Woolf.

In his 96th year, he published a new edition of his survey of world politics, characterised again by tart, far-sighted judgments. He would have made a superb chief of policy in the Foreign Office had his Greek surname not led Anthony Eden to steer him away.

Peter John Ambrose Calvocoressi, intelligence officer and writer: born Karachi 17 November 1912; married 1938 Barbara Eden (died 2005; two sons), 2006 Margaret Scott; died Devon 5 February 2010.